Given the increasing number of dramatic shortfalls in water supply, increasing usage of reverse osmosis (RO) to desalinate salt and brackish water has been made. In order for RO to be efficient, pretreatment of the feed water by processes such as filtration or clarification to remove particulate matter is typically necessary to prevent fouling of RO equipment, especially when the feed source is surface water. In order to determine the fouling potential of such water and the degree of pretreatment necessary to control RO fouling, the level of particulate matter in the feed water needs to be checked frequently. Such particulate level monitoring is conventionally done by an ASTM-specified manual test comprising measuring the rate of plugging of a 0.45 micron (micrometer) pore size membrane filter at 30 psi and calculating the plugging factor (PF) or silt density index (SDI) from the values obtained. However, this conventional manual method has a number of disadvantages. The method is time-consuming, requiring 30 minutes or more per analysis, and requires essentially the full attention of the test operator to time the filtration duration and volume measurements, monitor sample pressure and temperature, record data and perform calculations, and no quantitative PF or SDI values are available before the entire test is completed. An analysis response time of substantially less than 30 minutes would be highly desireable for the monitoring and adjustment of pretreatment performance for optimum particulate removal. Another major limitation of the ASTM test is that it is not recommended for waters with high plugging factors (&gt;75% after 5 minutes' filtration) which prevents the method's application to characterize feed water quality in many existing RO applications. The test apparatus and procedures are better suited for a laboratory environment than for the field. Failure of the operator to carefully install very thin membrane filters, which are best handled only by tweezers, into a filter holder and imprecise purging of all air bubbles from the apparatus before each measurement often lead to measurement errors and data scatter.
Existing automated monitors developed to perform PF and SDI tests also have drawbacks. Operator-intensive procedures of the manual method have been replaced by automated handling of membrane filters in the form of tape rolls, such as that disclosed in U.S. Pat. No. 4,554,822, requiring complicated, relatively bulky mechanical equipment which is impractical for field use and too expensive for most practical RO applications. Because membrane filter tape is not a standard product it is difficult to obtain a reliable supply of consistent quality at a reasonable price. Moreover, although such monitors automate the measurement process and data reporting, they do not increase the amount of data per sample nor improve upon data analysis procedures of the manual method. The automated PF monitor still requires about 20 minutes to complete one test, provides no PF data prior to test completion, cannot accurately analyze samples where the PF is very high, and provides no information on filter-plugging profiles, reflected in the shape of graphs of filtration flow rate versus filtration duration.
Existing filtration methods for monitoring the quality of ultrapure water such as are disclosed in U.S. Pat. Nos. 4,765,963 and 4,786,473 are unsuitable for measuring PF or SDI for RO feed water, which is typically at least two orders of magnitude more concentrated in particulate matter and ions than is ultrapure water. Such ultrapure water analyzers monitor continuously for changes in minute concentrations of submicro-sized particles (colloids) using a very fine filter having, for example, pores 0.1 micron in diameter, which are typically operated continuously with the same filter for days. A 0.1 micron filter operated on typical RO feedwater at 30 psi doesn't permit either sufficient flow or a representative sample of large particulate matter therethrough to provide any meaningful, reproducible quantitative data on level of particulate matter present. Water sampling techniques of ultrapure water monitors do not maintain sufficiently turbulent flow during sampling to maintain particles greater than 1.0 micron in suspension without some of them settling out in the apparatus before reaching the filtration step, especially when flow rate drops as a result of high levels of filter plugging. The location of a mechanical pressure regulator containing small flow passages in the sample pipe upstream of the filter exerts a filtering effect on larger particles, and the regulators' capability to maintain constant pressure is severely impaired as the filtration flow rate approaches zero. Particulate matter previously settled out from prior samples in the apparatus can contaminate subsequent samples, resulting in erroneously high PF readings. The required manual installation of membrane filters in holders results in the same errors discussed above pertaining to manual methods of measuring PF and SDI.
There is therefore a clear need in the art for a simple, reliable, semi-automated device for accurately measuring the level of suspended particulate matter in water, and that is capable of quickly reporting reproducible results.